Mobile hand held device makers are constantly looking for ways to differentiate their devices in the market. Current trends have focused on thinner displays, higher resolution, higher contrast, and increased display areas without making the device too large, etc. Most recently, there have been attempts to gain customer attention with curved displays or rounded display edges. With the adoption of organic light-emitting diodes (OLEDs), and in particular the emergence of flexible OLEDs, there is now a strong drive to produce devices that are foldable or rollable so that in addition to being compact, they can also be deployable to form a larger, continuous display.
One focus of current attempts to produce foldable or rollable devices is on making the devices less resistant to folding by segmenting or corrugating part of the structure in particular locations. Other attempts focus on ways to manage the position of the neutral plane(s) (i.e. the plane where tensile and compressive strain cancel each other, or in essence, where tensile and compressive strain is absent) as the multi-layer display is subjected to repeated folding and unfolding events. In these designs, the OLED layer is bonded to other layers of the device with significant efforts being made to position the OLED layer in the neutral plane and maintaining it in this plane during repeated flexing of the display. Due to the fragile nature of the OLED layer, significant effort is directed at designing the display stack so that the neutral plane is at, or at the very least, close to, the OLED layer. To help maintain the neutral plane in its required position, display stacks have been disclosed with so-called stress control members or elasticity adjusting layers (hereinafter “assembly layers”). In these cases, each of the individual layers of the display stack have been bonded together. As a result, individual film layers are now mechanically coupled and, for example, the bending stiffness of the composite structure (i.e., cover film/assembly layer/touch sensor layer/assembly layer/barrier layer/OLED layer/OLED substrate layer) is higher than if each layer were allowed to move independently from the others (as if there is no friction between them). As a result of being bonded, stresses and strains can also develop that may become increasingly significant as the modulus of the assembly layer increases.
FIG. 1 shows a typical cross-sectional view of a prior art flexible strain/stress-sensitive display 10. The prior art flexible strain/stress-sensitive 10 display generally includes a bottom layer or device housing 12 (“bottom housing 12”), a first assembly layer 14, an OLED module 16, a second assembly layer 18, a touch sensor 20, a third assembly layer 22, and a cover film 24. While not described here, other layers may be present in the display 10. For example, a circular polarizer or color filter may be positioned between the OLED layer or OLED module and the cover film. The OLED module 16 may generally include a flexible substrate 26, an OLED layer 28 and a flexible encapsulation layer 30. Although the OLED module is shown in FIG. 1 with only three layers, the OLED module 16 may include more layers. It has been demonstrated that these types of OLED modules (sometimes also called POLEDs for plastic OLEDs) are quite flexible and durable. However, to integrate this module into an actual device, it may have to be attached to a bottom layer 12 (for example an external driver circuit, a protective film, a flexible battery, etc.). The first assembly layer 14 can be optically clear, but it does not have to be as most POLEDs are top emitting. The top of the OLED module 16 may be attached to a flexible touch sensor 20, such as those based on polyester or cyclic olefin copolymer covered with an electro-conductive layer, such as a silver nanowire pattern. On top of the touch sensor 20 (or on top of the OLED module if the sensor is embedded or not used), a flexible cover film 24 is frequently applied. Between the OLED module 16 and the touch sensor 20, and between the touch sensor 20 and the cover film 24, are second and third assembly layers 18 and 22, respectively, applied to the whole surface of the display. The second and third assembly layers 18 and 22 are frequently optically clear adhesive layers. All of the assembly layers 14, 18 and 22 may also be an elasticity adjusting layer, a stress control member, or a stress redistribution layer. In addition to bonding the individual layers (bottom layer/OLED module/touch sensor/cover film) together, the assembly layers 14, 18 and 22 also need to fulfil the role of positioning the neutral plane in the stack in the right location (i.e. at or very close to the OLED layer 28, which may be damaged by even a slight amount of strain, compressive or tensile) and keeping it there during repeated bending of the display.
If any of the assembly layers 14, 18 and 22 deforms or creeps under the applied stress, there is a possibility that its thickness becomes non-uniform, resulting in optical distortions if the layer is in the path of the light emitted from the OLED module 16 or reflecting from it. Additionally, when a device including the OLED display 10 is turned off or closed for longer periods of time (i.e., overnight) or becomes hot (i.e., placed in a car exposed to sunlight in the summer time), the creep can become pronounced. Even if the assembly layer deformation is fully recoverable, excessively long relaxation times may make the image distortion too long-lasting. The assembly layers 14, 18 and 22 may also suffer from fatigue resulting from thousands of bending cycles occurring at different rates and different temperatures.